1. Field of the Invention
The present invention relates to an inspection method for a projection optical system, which requires highly accurate image formation characteristics, for fabricating semiconductor integrated circuits or liquid crystal devices, and also relates to an inspection apparatus for carrying out the inspection method and a projection exposure system provided with the inspection apparatus. The invention is applicable to a stepper type projection exposure apparatus but is particularly suitable for a projection exposure apparatus of the scan exposure type, such as a slit-scan type or step-and-scan type, where patterns of geometric shapes on a mask are serially transferred on a photosensitive substrate while scanning in synchronization the mask and the substrate.
2. Related Background Arts
Projection optical systems, which are mounted, for example, in a projection exposure apparatus for fabricating semiconductor integrated circuits or liquid crystal devices, require extremely high accuracy with respect to image formation characteristics such as projection magnification and image distortion. For this reason, a method for measuring projection magnification and image distortion of the projection optical system with high accuracy and a correction meth od for correcting the image formation characteristics with high accuracy have been developed. At present, there are roughly two methods for measuring projection magnification and image distortion.
The first method is one which transfers a pattern of a test mask to a photosensitive substrate (e.g., wafer). This method is disclosed, for example, in Japanese Patent Laid-Open Publication No. Sho 58-8353. In the first method, the test mask pattern is transferred onto a photosensitive substrate, and after the photosensitive substrate is moved a predetermined distance in accordance with a laser interferometer, the pattern is again transferred onto the substrate so that it overlaps with the previously transferred pattern. After development of the substrate, the overlap error or registration error is measured. At this time, since marks overlap one above the other after the photosensitive substrate is moved, the marks are patterns drawn at different places on the test mask.
The second method is one where an image of a pattern formed on a photosensitive substrate is measured directly by a photoelectric sensor without an actual exposure process such as the first method. This second method is disclosed, for example, in Japanese Patent Laid-Open Publication Nos. Sho 59-94032 or Sho 60-18738. An example of the second method will be briefly described in reference to FIGS. 1(a) and 1(b).
FIG. 1(a) shows a schematic structure of an example of a conventional projection exposure apparatus. As shown in FIG. 1(a), a test reticle TR as a test mask is provided with a plurality of light transmission portions 305A to 305B (in this example, slits) formed at predetermined intervals. Illumination light passes through the light transmission portions 305A to 305G and forms an image thereof on the photosensitive substrate side through a projection optical system PL. FIG. 1(a) shows the arrangement of a pattern plate 301 and a photoelectric sensor 302 (both of which are positioned on an image forming position of the light transmission portion 305A). The pattern plate 301 has a very small light transmission portion 306 which serves as a mark detection device. The photoelectric sensor 302 receives the illumination light from the light transmission portion 306. The pattern plate 301 and the photoelectric sensor 302 are mounted on a wafer stage, which has the photosensitive substrate mounted thereon and is movable on a plane perpendicular to an optical axis of the projection optical system PL. The position of the wafer stage is precisely measured by a reflector 303 fixed to the wafer stage and an external laser interferometer 304. The output of the photoelectric sensor 302 varies by scanning the wafer stage.
FIG. 1(b) shows a graph representing the result of the output of the photoelectric sensor 302. In the figure, the axis of abscissa represents a position x of the scanning direction of the wafer stage, and the axis of ordinate represents an output value I of the photoelectric sensor 302. The image forming position of the light transmission portion 305A of the test reticle TR can be measured by obtaining, in FIG. 1(b), a position x.sub.o where the output value I of an output curve 307 becomes maximum. If a similar measurement is performed with respect to a plurality of light transmission portions of the test reticle TR, the respective image forming positions of the light transmission portions 305A to 305G will be obtained. Therefore, the projection magnification and the image distortion of the projection optical system can be obtained because the positions of the light transmission portions 305A to 305G are known in advance.
In addition to the aforementioned photoelectric sensor scanning method, there is known a method where an image of a pattern on a reticle is magnified with a microscope and is detected by mean of an image pick-up device, such as two-dimensional CCD, or a method where, conversely, illumination light is emitted from a slit provided on a wafer stage and is received via a pattern of a test reticle TR by scanning the wafer stage (see Japanese Patent Laid-Open Publication No. Sho 63-81818).
The image formation characteristic of the projection optical system, such as magnification or image distortion, is required to be measured and regulated at the time of the manufacture by a projection exposure apparatus. The image formation characteristic also is required to be corrected at the time of actual use, because it varies due to an atmospheric pressure variation and illumination light absorption of a projection optical system. As a countermeasure, a method, where a quantity of the variation of the image formation characteristics are predicted in advance and correction of magnification is performed by varying an air pressure of the projection optical system, is known as disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 60-28613 or Sho 60-78457.
However, this method alone is insufficient and also there is the possibility that magnification and image distortion vary due to long-term fluctuations in a system. Therefore, the system should be used while periodically checking magnification and image distortion by means of methods such as described above. Also, since the demand for accuracy of correction corresponding to an atmospheric pressure variation has become increasingly severe in recent years, it is necessary to frequently check a correction error caused by measurement. In this sense, the aforementioned second method whose measurement time is short is superior to the first method, and in many cases, the second method is actually used.
Also, a step-and-scan projection exposure apparatus, where a mask and each shot area of a photosensitive substrate are scanned in synchronization with respect to a projection optical system in order to substantially increase an exposure area without greatly increasing the size of the projection optical system, has been aimed at in recent years. However, there has been provided no method for measuring an image formation characteristic, which makes use of, in particular, the feature of a projection exposure apparatus of a scan exposure type such as a step-and-scan type.
Thus, all of the aforementioned conventional methods of measuring the magnification or image distortion of the projection optical system are a method of measuring a gap or distance between the projected images of two (or more) different marks on a test mask. For this reason, it is necessary to accurately grasp the mark gap or pitch of the test mask in advance. However, normally the patterns on the mask are fabricated with an electronic beam drawing device, and the gap or pitch between spaced marks for measurement of magnification is not very accurate, so the gap of each mask has to be measured in advance with a reticle pattern measuring machine. This measurement is substantially impossible in a manufacturing site where a plurality of masks are used. For this reason, it is conceivable to use a reference mask, but this method has the disadvantage that measurement cannot be performed frequently during the aforementioned actual exposure.
Also, the above method has a problem regarding accuracy of reticle pattern measurement. For example, when a mask is mounted in an exposure apparatus, it is normally disposed with the pattern thereof facing downwardly, but in the reticle pattern measuring machine the mask is mounted with the pattern surface thereof facing upwardly. Therefore, influences of deflection caused by self-weight are different between the two masks. This difference results in a measurement error. In addition, even in the exposure apparatus, deflection by self-weight varies between the masks and causes image distortion.
Furthermore, although it is also conceivable that only some of the masks are measured in order to perform correction of the projection optical system, the gap or distance between the marks varies because the mask absorbs the heat of the illumination light during measurement and therefore the mask itself is thermally expanded. If correction, including the thermal expansion of the mask, is made during measurement, there will be no problem. However, if the mask is exchanged for a subsequent mask, there will be the drawback that an error remains in the magnification of the projection optical system, because the mask after exchange has not been thermally expanded. Moreover, even in a method where the distance between the marks is unknown but magnification is always held in an initial magnification obtained at the time of exchange, if a mask is exchanged for a subsequent one, an error will occur in the magnification of the projection optical system, for the same reasons.